This invention relates to a process for preparing a coal product from aqueous slurries of relatively fine and relatively granular coal particles. More particularly, the present invention relates to an improved procedure for conducting a process of the type described in U.S. Pat. No. 4,126,426.
Coal slurries may result from coal mining, coal transport or various processing carried out in order to upgrade the coal. In general, although quite large lumps (larger than say 25 mm) may be present, the majority of the particles are below 25 mm. Most slurries in fact comprise a substantial proportion of relatively fine particles which are smaller than about 100 microns, the remainder being relatively granular particles which are sized between about 100 microns and 25 mm. The choice of the limit of 100 microns here is made for convenience. In practice, the somewhat arbitrary dividing line between what are called fines and what are called grains is chosen somewhere in the range 75 to 250 microns. Those skilled in the art will bear this in mind in interpreting the present invention.
While the relatively granular particles in an aqueous slurry can be dewatered (that is to say separated from significantly more than the bulk of the water in which they are in suspension) by mechanical means, for example, screen filters or centrifuges, the relatively fine particles in an aqueous slurry cannot.
The term "agglomeration" as used herein refers to a process in which particles in an aqueous suspension are subjected to turbulence in the presence of a binder which is capable of wetting the surface of the particles and thus can cause the particles to stick together in clusters or so-called agglomerates. Selective agglomeration occurs when the binder preferentially wets certain solids. Those which are preferentially wetted, such as coal particles in the case of a hydrocarbon binder, are then agglomerated while those which are not, such as ash, remain in supension, U.S. Pat. No. 4,021,206 and corresponding United Kingdom application Nos. 5482/75 and 45156/75, filed Feb. 10 and Oct. 31, 1975, describe separating an aqueous slurry of mixed size coal particles by removing the larger particles (along with the water wetting their surfaces), agglomerating the remaining finer particles by agitating them with water and binder to form agglomerates which can be mechanically separated from the bulk of the liquid in which they were formed, then drying each of the products.
U.S. Pat. No. 4,126,426 and corresponding United Kingdom application No. 24836/77 filed June 14, 1977, describes a method of improving the products provided by U.S. Pat. No. 4,021,206 by using only about 8 to 10% by weight of the binder to form the agglomerates, mechanically dewatering the agglomerates, then mixing the agglomerates with the previously removed larger particles of coal.
Netherlands patent application No. 7807223 filed July 4, 1978, describes agglomerating an aqueous slurry of mixed size coal particles by agitating the mixed particles with water and a binder to yield mechanically-dewaterable agglomerates, dewatering those agglomerates and re-treating them with hot water and relatively viscous binder (to form hot agglomerates which are relatively hard) while recycling the unagglomerated coal particles back to the first agglomerating zone.
The above patents and applications and the references cited regarding the U.S. patents are believed to comprise the most pertinent references known to the applicant. The disclosures of the U.S. Pat. Nos. 4,021,206 and 4,126,426 are incorporated herein by cross reference.